For microbiological laboratory test purposes, it is known to use flat dishes of glass or translucent plastic, which are coated with a thin, uniform gel layer. The substance-coated carriers are placed onto this layer and the diffusion of the substance contained in the carriers takes place into this gel layer. Depending upon the test and the substance, the diffiusion results in an inhibition or promotion of the growth of the microorganisms placed upon the gel layer.
The diameter of growth inhibition or growth promotion zones is a measure for the outcome of the test, which in turn makes it possible to reach conclusions as to the utility of a substance as an antibiotic or as to the nature of a microorganism.
As carriers for test substances, round slips of filter paper of small diameter have generally established themselves in practice. The usual methods for applying the individual slips onto the surface of the gel can be classified into three groups:
(A) applying the slips individually with a tweezers and pressing the slips individually against the surface of the gel;
(B) applying a group of slips, which are interconnected spatially with bridges of filter paper, with a tweezers and pressing them individually against the gel surface; and
(C) applying a group of carriers with the aid of a mechanical dispensing device which has a plurality of supply containers with slips and causes one slip at a time to drop from each supply container by means of pressure exerted onto a handle and then presses them against the gel surface.
Reading out the test result, which is expressed by a more or less pronounced inhibition or promotion of the growth of the microorganisms in a circle around the point at which they were applied, is at the present time performed in the following ways:
(1) By estimating the diameter of the circular field of growth; this is the most often-used method.
(2) By measurement with a ruler or slide gauge directly or following projection onto a screen.
(3) By comparison with previously prepared, movable scales which are held above the gel or, in the case of a translucent gel, under it.
In cases (A) and (B) above, the application of the slips of paper is associated with numerous manipulations, which impair both the precision with which the slips are placed in position and the rapidity with which this can be accomplished. In case (C), a precondition is an apparatus which involves investment costs and requires a person experinced in its operation. Because it is also necessary to keep relatively large quantities of substance carriers on hand, this manner of applying the slips is suitable only for laboratories with a relatively large volume of work.
Reading out the results by estimation as in category (1) above requires extensive experience and yet is still not reliable.
Using standard measuring rods as in category (2) above, while more precise and producing better results, still is hardly ever used because of the possibility of unintended displacements, parallax errors and time losses. Comparison with previously prepared, movable scales as in (3) is again not entirely reliable because of the danger of unintended displacement and parallax during the reading of the results and is also not free from the necessity for manipulation; in other words, it causes lost time.